So much for Arlen Specter’s party switch to avoid a risky primary. The incumbent Pennsylvania senator’s 2010 Democratic Primary race against challenger Joe Sestak is now a toss-up.

A new Rasmussen Reports telephone survey of likely Democratic Primary voters in the state finds Specter with just a four-point lead over Sestak – 46% to 42%. Two percent (2%) prefer some other candidate, and 10% are undecided.

In early August, Specter had a 13-point lead over the two-term congressman from the Philadelphia suburbs. In June, Sestak trailed by 19 points.

Support for Specter in the primary has stayed relatively stable, declining just five points since June. However, support for Sestak has increased by 10 percentage points in recent months.

Sestak runs even with likely Republican candidate Pat Toomey in an early look at the 2010 general election Senate contest in Pennsylvania, while Specter is now five points behind the man he once feared would beat him in next year’s GOP Primary.

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Specter, a Republican senator for nearly 30 years, switched parties in April just after a Rasmussen Reports poll showed him trailing Toomey by 21 points. He acknowledged becoming a Democrat in part out of fear of losing the party primary.

President Obama, Pennsylvania Governor Edward Rendell and other top Democrats welcomed Specter because it strengthened their party’s majority in the Senate and promised to campaign for his reelection. But Sestak, a retired Navy admiral, opted for a primary challenge of the longtime senator, campaigning on the platform that he is the “real Democrat” in the race.

Interestingly, favorables for the two men are basically unchanged from August. Thirty percent (30%) have a very favorable opinion of Specter, while just 14% view Sestak very favorably.

Specter is seen very unfavorably by 17%. Eight percent (8%) say the same of Sestak. Thirty percent (30%) of likely primary voters don’t know enough about Sestak to even venture a soft opinion of him.

The upcoming Senate health care vote could prove crucial for Specter. Sixty-six percent (66%) of Democratic Primary voters favor the so-called “public option” as a part of the health care plan proposed by President Obama and congressional Democrats. Only 21% oppose the creation of a government-run health insurance company to compete with private insurers.

But the “public option” runs dead even when put to all Pennsylvania voters.

Support for the “public option” drops to 37% among Democratic Primary voters if the creation of the government plan might encourage some companies to drop health insurance coverage for their employees. Forty-five percent (45%) are opposed under that scenario.

Sixty-three percent (63%) of all Pennsylvania voters oppose the “public option” if it encourages some companies to drop their existing health insurance coverage. Only 23% of state voters support it at that point.

Obama held a fundraiser for Specter last month and is expected to campaign for him in Pennsylvania next year. Right now, that gives Specter an important edge since 79% of Democratic Primary voters approve of the president’s job performance, including 55% who strongly approve. Just 21% disapprove, with 13% who strongly disapprove.

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